Doped fiber amplifiers such as erbium-doped fiber amplifiers (EDFA's) are expected to play a key role in the implementation of high-capacity, high-speed fiber-optic communication networks.
The average output signal power of an EDFA increases monotonically in a non-linear manner with the average input signal power.
The roll-over in the input/output characteristics of the EDFA is due to gain saturation effects. This is a desirable feature providing a weak (soft) optical limiting of the output power for a wide range of input signals.
However, in a variety of applications, it is imperative for the output signal power of the amplifier to be always constant (hard limiting) at particular points of the optical network. In these cases, the generic saturation non-linearity of the EDFA is not sufficient to limit the amplifier output power.
This task is accomplished by an EDFA with special input-output characteristics, called optical power limiting amplifier (OLA). The output signal powers remains substantially constant (i.e. varies less than .+-.0.5 dB) over a range of input signal power, called the dynamic range of the OLA. In addition, a hard limiting amplifier will be useful in reducing the dynamic range requirements for the electronic and optical components in the network.
Optical power limiting amplifiers are known from W. I Way et al., in Electronics Letters, Jan. 31, 1991, vol. 27, no. 3, pp. 211-213, and in Journal of Lightwave Technology, vol. 10, no. 2, February 1992, pp. 206-214.
In the first paper a limiting optical amplifier has been demonstrated using three cascaded stages of erbium doped fiber amplifiers, each pumped with respective pump sources arranged to counterpropagate with respect to the signal; a Fabry-Perot filter is placed between first and second stages and an angle tuned Fabry-Perot etalon is placed between the second and the third stages.
An optical isolator is placed after each stage of amplifier to avoid optical reflections.
In the second paper an OLA was built using two cascaded stages of fiber amplifiers, two optical isolators and two optical bandpass filters.
Both the above solutions make use of filtering means located after the amplifier, or after each stage of amplifier; the filters introduce relevant overall loss in the signal, of about 9-12 dB.
The optical isolators are used after the end of each amplifier and are used to avoid optical reflections.
In EP 93300762.7 and in Proceedings of Optical Amplifier Topical Meeting, Santa Fe, N.M., June 1992, Paper FB2, pp. 162-165, it is disclosed the use of an isolator inserted within two lengths of erbium doped fiber of an optical amplifier, to the purpose of reducing the backward travelling amplified spontaneous emission (ASE); the optimum isolator position range is found at 30%=15% of the total EDFA length.
In U.S. Pat. No. 5,050,949 a multi stage optical fiber amplifier is shown, which provides gain spectrum equalization for channels at different wavelengths, by means of a two stage fiber amplifier with two doped fiber compositions, to obtain, for each stage, a different gain spectrum; an optical isolator is introduced between the two stages to suppress reflection induced noise and spontaneous emission.
In this amplifier no power limiting effect is shown at a single signal wavelength.
In EP 0 470 497 an optical fiber amplifier is shown, where a coil-formed erbium doped optical fiber is used, with a radius of curvature set such that the bend loss caused on the signal light with the wavelength 1.536 .mu.m is small, while the bend loss caused on the light generated by erbium spontaneous emission, with the wavelength 1.55 .mu.m, is great; the preferred bend radius is 20 mm and the gain markedly decreases when the radius of curvature is 13 mm or below.
No power limiting effect is shown at the signal wavelength and no differential loss is taught between signal and pump light.